The Cognitive and Neural Architecture of Semantic Cognition: Evidence for Dissociable Distributed Systems from Multiple Methods

This thesis aimed to dissociate temporoparietal contributions to semantic cognition and investigate the wider semantic control network using behavioural experiments, fMRI, and TMS. Chapter 2 investigated patients with semantic aphasia (SA) and healthy participants under conditions of divided attention and found that the selection of associative knowledge, specifically for weaker associations is reliant on semantic control processes. Chapter 3 utilised TMS to dissociate two sites in the temporoparietal region implicated in semantic cognition (posterior middle temporal gyrus, pMTG; and angular gyrus, AG), which are co-activated in semantic contrasts and often damaged together in SA. pMTG was involved in semantic control whereas the response in AG suggests that it is involved in reflexive orientation to semantic concepts. Chapter 4 examined whether the network involved in the control of semantic information overlapped with the network involved in action selection, as both semantic selection and action selection activate overlapping regions demonstrated by previous fMRI studies. Significant overlap was observed between semantic control and action selection in the left inferior frontal gyrus (LIFG) and pMTG suggesting that flexibility in semantic retrieval and action selection rely on partially overlapping architecture. In Chapter 5 we extended previous work demonstrating an involvement of different parts of LIFG in different aspects of semantic control. Dorsal LIFG showed engagement in goal-driven selection while anterior ventral LIFG showed a response compatible with flexible content-driven retrieval. This distinction extended to posterior temporal cortex with pMTG recruitment only observed for context-driven retrieval demands and ITC involvement in goal-driven semantics. The findings of this thesis further elucidate the role of distinct regions within temporoparietal cortex in semantic cognition and the apparent overlap between semantic control and event/action understanding.